1. Field of the Invention
The present invention relates generally to a high-density optical recording medium system and, more particularly, to an apparatus and method for reproducing records for optical recording medium, capable of detecting and compensating for detrack, tilt and defocus of the optical recording medium.
2. Description of the Related Art
In general, examples of optionally and iteratively rewritable optical recording medium include rewritable compact disc (CD-RW) and rewritable digital versatile disc (DVD-RW, DVD-RAM, DVD+RW).
These rewritable optical discs, particularly, DVD-RAMs have signal tracks made up of lands and grooves and enable the tracking control of an empty disc on which no information signal is written. Recently, information signals are also written on the tracks of lands and grooves so as to enhance recording density. For this purpose, the recent optical pickup for writing and reading information signals uses the shorter wavelength of laser beam with an increased number of apertures formed in the object lens and thereby reduces the size of beam for writing/reading records.
In order to achieve higher recording density, such a rewritable high-density optical disc is designed to have a reduced distance between the signal tracks, i.e., the smaller signal track pitch.
For the rewritable discs, it is naturally impossible to perform a disc control and a recording operation in an empty disc in which no information is written. Thus disc tracks are formed in lands and grooves to write information on, and control information for random access and rotation control is separately recorded in the disc, so as to enable tracking control in the empty disc.
The control information is, as shown in FIG. 1, written on the header pre-formatted at the beginning position of each sector, or along the track in the wobbling profile. The term xe2x80x9cwobblingxe2x80x9d as used herein refers to recording the control information on the boundary of tracks in accord to variation of laser beam by supplying power of laser diodes with information for modulating a predetermined clock and applying the modulated clock to the disc, e.g., information about a desired position and the rotational speed of the disc.
In a DVD-RAM, the header preformatted at the beginning position of each sector includes four header fields, e.g., header 1 field, header 2 field, header 3 field and header 4 field. Each header field has variable frequency oscillator (VFO) areas for generating a reference clock to acquire bit synchronization of read channels. In the present invention, the VFO areas present in the respective header fields (header 1 fieldxcx9cheader 4 field) are called VFO1xcx9cVFO4.
That is, VFO1 and VFO 3 areas are present in the header 1 field and the header 3 field, VFO2 and VFO4 areas being in the header 2 field and the header 4 field. The VFO1 and VFO3 areas are longer and more stable for signal detection than the VFO2 and VFO4 areas.
The four header fields are staggered with respect to each other from the track center. FIG. 1 shows an example of the header for the first sector in a track. Referring to FIG. 1, the track boundary of the user area in which data are actually written has a wobbling profile.
An optical record reproducing apparatus also performs tracking and focus controls with an optical pickup in writing and reading information.
That is, tracking control, e.g., tracking servo involves detection of tracking error signals from electrical signals generated in accordance to the beam trace status and driving a tracking actuator in the optical pickup based on the tracking error signals to move an object lens of the optical pickup in the radial direction, thereby changing the position of the beam to trace a desired track.
There are some cases where detrack occurs that the beam focus is deflected from the track center, even though no tracking error signal is detected. Detrack does not adversely affect the compact discs.
However, detrack has an adverse effect on the optical discs such as DVD-RAM where data writing and reading is enabled in both lands and grooves, because the track pitch is narrowed for purpose of high densification.
Due to a depth difference between lands and grooves, detrack may occur in the tracks of the grooves even when no detrack is detected in the tracks of the lands. Likewise, the tracks of the lands may have detrack while there is no detrack detected in the tracks of the grooves.
If detrack occurs, writing/reading data becomes harder because the beam is ready to shift to the adjacent track to cause a cross talk and clear data from the track.
In a case where the beam focus is deflected from the disc surface during a focus control, i.e., focus servo, which case will be referred to as xe2x80x9cdefocusxe2x80x9d hereinafter, quality of data deteriorates in writing and reading the data and thereby the system operation becomes unstable.
The focus servo drives a focus actuator in the optical pickup to move the optical pickup up or down and make the beam in focus according to the turning and up-and-down motions of the optical disc. That is, the focus actuator drives the object lens for convergence of beam in the upward/downward direction, e.g., in a direction of the focus axis to maintain a constant distance between the object lens and the optical disc.
However, in the optical discs such as DVD-RAM where data can be written in both lands and grooves, the lands and grooves differ in the focus offset from each other due to a depth difference and cause defocus even when no focus error signal is detected.
That is, due to the depth difference between the lands and grooves, defocus may be detected in the tracks of the grooves even when no detrack occurs in the tracks of the lands. Likewise, the tracks of the lands may have detrack while detrack is not detected in the tracks of the grooves.
As the defocus status cannot be known only from the focus error signals in this case, jitter characteristic deteriorates and the bit error rate (BER) increases. Recording data in this state may result in change recording characteristics of lands and grooves and hence deterioration of data quality, which makes the system operation unstable.
During a resin extracting and hardening process in fabrication of the optical disc, distortion may take place in the optical disc and cause eccentricity even when a central aperture is perforated in optical disc. Also, deviation of the central aperture causes eccentricity although the tracks of the disc are accurately provided in the radial form with a defined pitch. Thus as the disc turns with eccentricity, the central axis of the motor is not in perfect accord with the center of the track.
It is thus hard to read out the signals of a desired track only. So, in the CD and DVD systems, a tracking servo is performed according to the standards established for the deflected quantity such that the beam always traces the desired track in spite of eccentricity.
It means, the tracking servo generates electrical signals corresponding to the beam trace status and moves the object lens or the optical pickup body in the radial direction based on the generated electrical signals, to change the position of the beam and make the beam trace the accurate track.
Meanwhile, the beam can be deflected from a desired track due to a tilt of the disc as well as the eccentricity. This results from a mechanic error occurring when the disc is set on a spindle motor. That is, the focusing direction is not in perpendicular relation with the tracking direction. This slant state of the disc is called xe2x80x9ctiltxe2x80x9d.
Tilt is not so significant for compact discs that have a large tilt margin due to their wide track pitch. The term xe2x80x9ctilt marginxe2x80x9d as used herein refers to a compensable quantity of tilt of the disc. However, with a growing need of densification of the optical appliances such as optical discs, especially in the DVD having the narrower track pitch, a slight tilt of the disc causes the beam to shift to the adjacent track due to a small radial tilt margin for the jitter. This xe2x80x9cdetrackxe2x80x9d is unavoidable by the tracking servo only. That is, the tracking servo may mistake that the beam is tracing the accurate track even when the beam is shifted to the adjacent track due to tilt, while focusing on the center of the track.
This makes it impossible to write/read data in/from a desired track. Thus a double distortion occurs when reading the erroneously written data.
To cope with this problem, there has been suggested a method in which the tilt of the disc can be detected with a dedicated tilt sensor, e.g., a tilt light-receiving device in an optical pickup. However, the method is not so efficient with a large size of the set.
It is, therefore, an object of the present invention to provide an apparatus and method for reproducing records for an optical recording medium, capable of detecting and compensating for detrack from header areas staggered with respect to each other.
It is another object of the present invention to provide an apparatus and method for reproducing records for an optical recording medium, capable of detecting and compensating for tilt from header areas staggered with respect to each other.
It is further another object of the present invention to provide an apparatus and method for reproducing records for an optical recording medium, capable of detecting and compensating for defocus from header areas staggered with respect to each other.
It is still another object of the present invention to provide an apparatus and method for reproducing records for an optical recording medium, capable of iteratively controlling detrack, tilt and defocus in a predefined order.
To achieve the above objects of the present invention, there is provided a method for reproducing records for optical recording medium includes the steps of: (a) determining a difference between a difference signal of optical reflecting signals of the optical recording medium detected at the non-writable area and the center level at an adjacent data area to output a first signal; (b) determining a difference between a difference signal of optical reflecting signals of the optical recording medium detected at a second non-writable area and the center level at an adjacent area to output a second signal, the second non-writable area being different in phase from the non-writable area; (c) determining a difference between the first signal and the second signal to output a variation; (d) comparing the variation with a predetermined threshold, determining that detrack has occurred, if the variation exceeds the threshold, and outputting the resulting value; and (e) performing a tracking servo based on the resulting value.
The difference signal between the optical reflecting signals includes a read channel 2 signal generated from electrical signals output in proportion to the quantity of beam reflected from the optical recording medium.
A read channel 1 signal means the total output of the split photo detectors. The read channel 2 signal means the differential output of the split photo detectors.
The difference signal between the optical reflecting signals includes a tracking error signal obtained by filtering the read channel 2 signal generated from electrical signals output in proportion to the quantity of beam reflected from the optical recording medium.
The tracking servo step (e) detects the magnitude and the direction of detrack from the resulting value and the sign of the variation, respectively.
The tracking servo step (e) performs the tracking servo in such a manner as to equalize the level of the first signal to the level of the second signal.
The tracking servo step (e) performs the tracking servo in such a manner that two tracking error signals of different phases is in symmetric relation with each other with respect to the center level of the adjacent data area.
In another aspect of the present invention, a method for reproducing records for optical recording medium includes the steps of: (a) determining a difference between a difference signal of optical reflecting signals of the optical recording medium detected at the non-writable area and the center level at an adjacent data area to output a first signal; (b) determining a difference between a difference signal of optical reflecting signals of the optical recording medium detected at a second non-writable area and the center level of an adjacent data area to output a second signal, the second non-writable area being different in phase from the non-writable area; (c) determining a difference between the first signal and the second signal to output a variation; (d) comparing the variation with a predetermined threshold, determining that tilt has occurred, if the variation exceeds the threshold, and outputting the resulting value; and (e) performing a tilt servo based on the resulting value.
The tilt servo step (e) detects the magnitude and the direction of tilt from the resulting value and the sign of the variation, respectively.
The tilt servo step (e) performs the tilt servo in such a manner as to equalize the level of the first signal to the level of the second signal.
The tilt servo step (e) performs the tilt servo in such a manner that two tracking error signals of different phases is in symmetric relation with each other with respect to the center level of the adjacent data area.
In further another aspect of the present invention, a method for reproducing records for optical recording medium includes the steps of: (a) determining, when no tilt is detected, a first potential difference between a ground level and a read channel 2 signal detected at the non-writable area, and setting the potential difference as a reference value; (b) determining, when necessary, a second potential difference between a second ground level and a second read channel 2 detected at the non-writable area, and comparing the first potential difference with the second potential difference; and (c) determining from the result of the comparison in step (b) that tilt has occurred, and performing a tilt servo.
In still further another aspect of the present invention, a method for reproducing records for optical recording medium includes the steps of: (a) determining a difference signal between optical reflecting signals each detected at the plural non-writable areas of different phases to output a variation; (b) comparing the variation with a predetermined threshold, determining that defocus has occurred, if the variation exceeds the threshold, and outputting the resulting value; and (c) performing a focus servo based on the resulting value.
In the variation outputting step (a), a peak-to-peak voltage of read channel 1 signals or read channel 2 signals detected at the non-writable areas is a first signal, and a peak-to-peak voltage of read channel 1 signals or read channel 2 signals detected at a second non-writable areas is a second signal, the second non-writable areas being different in phase from the non-writable areas, the variation being the difference between the first signal and the second signal.
The focus servo step (c) is performed in such a manner that the sum signal of the first and second signals is at maximum and the variation does not exceed the threshold.
The variation outputting step (a) includes the steps of: determining a potential difference between the tracking error signal detected at the non-writable area and the track center level of an adjacent data area to output a first signal; determining a potential difference between the tracking error signal detected at another non-writable area and the track center level of an adjacent data area to output a second signal; and determining a difference between the first and second signals as the variation.
The focus servo step (c) detects the magnitude and the direction of defocus from the resulting value and the sign of the variation, respectively.
The focus servo step (c) performs the focus servo in such a manner as to equalize the level of the first signal to the level of the second signal.
In still further another aspect of the present invention, a method for reproducing records for optical recording medium includes the steps of: (a) detecting a first variation from a first potential difference between a difference signal of optical reflecting signals each detected at the plural non-writable areas of difference phases and a first reference level, and detecting and compensating detrack of the optical recording medium from the first variation; (b) detecting a second variation from a second potential difference between a difference signal of optical reflecting signals each detected at the plural non-writable areas of difference phases and a second reference level, and detecting and compensating tilt of the optical recording medium from the second variation; and (c) detecting a third variation from a third potential difference between a difference signal of optical reflecting signals each detected at the plural non-writable areas of difference phases and a third reference level, and-detecting and compensating defocus of the optical recording medium from the third variation, wherein the detecting and compensating steps are performed in the order of detrack, tilt and defocus.
In still further another aspect of the present invention, an apparatus for reproducing records for optical recording medium includes: a signal generator for generating a difference signal between optical reflecting signals from electrical signals generated from an optical pickup for recording/reproducing information on/from the optical recording medium; a detrack detector for detecting detrack of the optical recording medium from a variation of the difference signal between the optical reflecting signals of the non-writable areas output from the signal generator, and outputting a detrack error signal; a tilt detector for detecting tilt of the optical recording medium from a variation of the difference signal between the optical reflecting signals of the non-writable areas output from the signal generator, and outputting a tilt error signal; a defocus detector for detecting defocus of the optical recording medium from a variation of the difference signal between the optical reflecting signals of the non-writable areas output from the signal generator, and outputting a defocus error signal; a servo controller for generating a tracking driving signal from the detrack error signal detected at the detract detector, a tilt driving signal from the tilt error signal detected at the tilt detector, and a focus driving signal from the defocus error signal detected,at the defocus detector; a tracking driver for controlling the optical pickup based on the tracking driving signal to compensate for detrack; a tilt driver for controlling the optical pickup based on the tilt driving signal to compensate for tilt; and a focus driver for controlling the optical pickup based on the focus driving signal to compensate for defocus.